Cr-W-V bainitic / ferritic steel compositions

ABSTRACT

A high-strength, high-toughness steel alloy includes, generally, about 2.5% to about 4% chromium, about 1.5% to about 3.5% tungsten, about 0.1% to about 0.5% vanadium, and about 0.05% to 0.25% carbon with the balance iron, wherein the percentages are by total weight of the composition, wherein the alloy is heated to an austenitizing temperature and then cooled to produce an austenite transformation product.

[0001] The United States Government has rights in this inventionpursuant to contract no. DE-AC05-000R22725 between the United StatesDepartment of Energy and UT-Battelle, LLC.

FIELD OF THE INVENTION

[0002] The present invention relates generally to ferritic steel alloysand, more specifically, to a high-strength, high-toughness Cr—W—Vferritic steel alloy having a bainite microstructure achieved throughthe alloy composition and by controlling the cooling rate from anaustenitizing temperature.

BACKGROUND OF THE INVENTION

[0003] Cr—W—V bainitic/ferritic steel compositions are of interest forhigh-strength and high-toughness applications. Please see U.S. Pat. No.5,292,384 issued on Mar. 8, 1994 to Ronald L. Klueh and Philip J.Maziasz, entitled “Cr—W—V bainitic/ferritic steel with improved strengthand toughness and method of making”, the entire disclosure of which isincorporated herein by reference.

[0004] There is usually a trade off in strength and toughness for mostengineering materials: improved toughness usually comes at the expenseof strength. The new ferritic steels have a bainite microstructure, andbainitic steels are generally used in the normalized-and-tempered orquenched-and-tempered conditions. Normalizing involves ahigh-temperature austenitizing anneal above the A_(C3) temperature (thetemperature where all ferrite transforms to austenite on heating) and anair cool, and quenching involves the austenitization anneal and a waterquench; tempering involves a lower-temperature anneal—below the A_(C1)temperature (the temperature at which ferrite begins to transform toaustenite on heating). Tempering at higher temperatures and/or longertimes at a given temperature improves the toughness at the expense ofstrength.

[0005] The objective, therefore, is to develop steels with optimizedstrength and toughness. Ideally, such steels would develop a lowductile-brittle transition temperature (DBTT) and high upper-shelfenergy (USE) with minimal tempering (i.e., tempering at a lowtemperature or for a short time), thus allowing for high-strength andtoughness. An ideal bainitic steel composition is one that can beproduced by normalizing (air cooling) or quenching in water or othercooling media and then could be used without tempering. Economicconsiderations have made such steels a goal of the steel industry.

[0006] Early work on Fe-2.25Cr-2.0W-0.25V-0.1C (2{fraction (1/4)}Cr-2WV) demonstrated that by a proper heat treatment of Fe—Cr—W—V—C steels,it was possible to produce two different bainitic microstructures, shownin FIGS. 1a and 1 b, in the normalized-and-tempered condition. It wasdiscovered that the normalized-and-tempered microstructures developedduring tempering were from two different bainite microstructures thatformed during normalization; they were: carbide-free acicular bainiteand granular bainite. The large blocky carbide particles thatprecipitate in the granular bainite are probably responsible for theinferior toughness of this steel.

[0007] Carbide-free acicular bainite consists of thin sub-grainscontaining a high dislocation density with an acicular appearance, shownin FIG. 2a. Granular bainite has an equiaxed appearance with bainiticferrite regions of high dislocation density and dark regions, shown inFIG. 2b. The dark regions have been determined to be martensite andretained austenite and have been called “M-A islands”(martensite-austenite islands). They form because during the formationof the bainitic ferrite, carbon is rejected into the untransformedaustenite. The last of the high-carbon austenite regions are unable totransform to bainite during cooling. Therefore, parts of thesehigh-carbon regions transform to martensite when the steel is cooledbelow the martensite start (M_(s)) temperature. The remainder is presentas retained austenite.

[0008] Whether carbide-free acicular bainite or granular bainite formduring the normalization heat treatment depends on the cooling rate fromthe austenitization temperature. The difference in microstructure can beexplained using a continuous-cooling diagram, shown in FIG. 3 (see forexample, L. J. Habraken and M. Economopoulos, Transformation andHardenability in Steels, Climax-Molybdenum Company, Ann Arbor, Mich.,1967, p. 69, R. L. Klueh and A. M. Nasreldin, Met. Trans. 18A, 1987, p.1279; R. L. Klueh, D. J. Alexander, and P. J. Maziasz, Met. Trans. 28A,1997, p. 335). If the steel is cooled rapidly enough to pass throughZone I in FIG. 3, acicular bainite forms; if cooled more slowly throughZone II, granular bainite forms.

[0009] Mechanical properties studies of the different bainites indicatedthat the acicular bainite had superior strength and toughness comparedto the granular bainite. As an alternative to an increased cooling rateto achieve the favorable properties, it was concluded the same effectcould be obtained if the hardenability was increased. To increasehardenability, the chromium and tungsten compositions were increased,and acicular bainite could then be produced in a 3Cr-2WV and 3Cr-3WVsteel, whereas granular bainite was always produced for similar heattreatment conditions in the 2{fraction (1/4)}Cr-2 WV steel, as shown inFIG. 4.

OBJECTS OF THE INVENTION

[0010] Accordingly, objectives of the present invention includeprovision of Cr-W-V bainitic/ferritic steel compositions that do notrequire a temper and/or post-weld heat treatment prior to use. Furtherand other objectives of the present invention will become apparent fromthe description contained herein.

SUMMARY OF THE INVENTION

[0011] In accordance with one aspect of the present invention, theforegoing and other objects are achieved by a high-strength,high-toughness steel alloy includes about 2.5% to about 4% chromium,about 1.5% to less than 2% tungsten, about 0.1% to about 0.5% vanadium,and about 0.05% to 0.25% carbon with the balance iron, wherein thepercentages are by total weight of the composition, wherein the alloy isheated to an austenitizing temperature and then cooled to produce anaustenite transformation product.

[0012] In accordance with another aspect of the present invention, ahigh-strength, high-toughness steel alloy includes about 2.5% to about4% chromium, about 1.5% to about 3.5% tungsten, greater than 0.3% toabout 0.5% vanadium, and about 0.05% to 0.25% carbon with the balanceiron, wherein the percentages are by total weight of the composition,wherein said alloy is heated to an austenitizing temperature and thencooled to produce an austenite transformation product.

[0013] In accordance with a further aspect of the present invention, amethod of producing a high-strength, high-toughness steel compositionincludes the steps of: forming a body of a ferritic steel compositioncomprising about 2.5% to about 4% chromium, about 1.5% to less than 2%tungsten, about 0.1% to about 0.5% vanadium, and about 0.05% to 0.25%carbon with the balance iron, wherein the percentages are by totalweight of the composition; heating the composition to an austenitizingtemperature for a predetermined length of time; and cooling thecomposition from the austenitizing temperature at a rate to form anaustenite transformation microstructure.

[0014] In accordance with a further aspect of the present invention, amethod of producing a high-strength high-toughness steel compositionincludes the steps of: forming a body of a ferritic steel compositioncomprising about 2.5% to about 4% chromium, about 1.5% to about 3.5%tungsten, greater than 0.3% to about 0.5% vanadium, and about 0.05% to0.25% carbon with the balance iron, wherein the percentages are by totalweight of the composition; heating the composition to an austenitizingtemperature for a predetermined length of time; and cooling thecomposition from the austenitizing temperature at a rate to form anaustenite transformation microstructure.

[0015] In accordance with a further aspect of the present invention, amethod of producing a high-strength, high-toughness steel alloy includesthe steps of: forming a body of a ferritic steel composition comprising2.5% to 4.0% chromium, 1.5% to less than 2% tungsten, 0.0% to 1.5%molybdenum, 0.10% to 0.5% vanadium, 0.2% to 1.0% silicon, 0.2% to 1.5%manganese, 0.0% to 2.0% nickel, 0.0% to 0.25% tantalum, 0.05% to 0.25%carbon, 0.0% to 0.01% boron, 0.0% to 0.2% titanium, 0.05% to 0.25% Nb,0.0 to 0.08% nitrogen, 0.0% to 0.2% Hf, 0.0% to 0.2% Zr, and 0.0 to0.25% Cu, with the balance iron, wherein the percentages are by totalweight of the composition; heating the composition to an austenitizingtemperature for a predetermined length of time; cooling the compositionat a rate to form a carbide-free acicular bainite microstructure; andtempering the composition at a temperature of not more than about 760°C. for a time of up to 1 hour per inch of thickness of the composition.

[0016] In accordance with a further aspect of the present invention, amethod of producing a high-strength, high-toughness ferritic steel alloyincludes the steps of: forming a body of a ferritic steel compositioncomprising 2.5% to 4.0% chromium, 1.5% to 3.5% tungsten, 0.0% to 1.5%molybdenum, greater than 0.3% to 0.5% vanadium, 0.2% to 1.0% silicon,0.2% to 1.5% manganese, 0.0% to 2.0% nickel, 0.0% to 0.25% tantalum,0.05% to 0.25% carbon, 0.0% to 0.01% boron, 0.0% to 0.2% titanium, 0.05%to 0.25% Nb, 0.0 to 0.08% nitrogen, 0.0% to 0.2% Hf, 0.0% to 0.2% Zr,and 0.0 to 0.25% Cu, with the balance iron, wherein the percentages areby total weight of the composition; heating the composition to anaustenitizing temperature for a predetermined length of time; coolingthe composition at a rate to form a carbide-free acicular bainitemicrostructure; and tempering the composition at a temperature of notmore than about 760° C. for a time of up to 1 hour per inch of thicknessof the composition.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1a. is a photomicrograph of tempered structures ofcarbon-free acicular bainite in 2¼Cr-2WV steel.

[0018]FIG. 1b is a photomicrograph of tempered structures of granularbainite in 2¼Cr-2WV steel.

[0019]FIG. 2a is a photomicrograph of the 2¼Cr-2WV steel after a slowcool from the austenitization temperature.

[0020]FIG. 2b is a photomicrograph of the 2¼Cr-2WV steel after a fastcool from the austenitization temperature.

[0021]FIG. 3 is a schematic representation of a continuous-coolingtransformation (CCT) diagram.

[0022]FIG. 4a is a photomicrograph of normalized 3Cr-2WV steel with thedesired acicular bainite achieved by increasing hardenability over thatof the 2¼Cr-2WV.

[0023]FIG. 4b is a photomicrograph of normalized 3Cr-3WV steel with thedesired acicular bainite achieved by increasing hardenability over thatof the 2¼Cr-2WV.

[0024]FIG. 5 is a graph showing effects of varying the molybdenumcomposition on the DBTT of various steels.

[0025]FIG. 6 is a graph of creep-rupture properties of the 3Cr-3WV and3Cr-3WVTa steels at 600° C. in the normalized andnormalized-and-tempered conditions compared to three commercial steels.

[0026]FIG. 7 is a graph of creep-rupture properties of the 3Cr-3WV and3Cr-3WVTa steels at 650° C. in the normalized andnormalized-and-tempered conditions compared to a commercial steel.

[0027]FIG. 8 is a graph of Rockwell hardness of 3Cr-3WV base (V alloys)with various compositional variations.

[0028]FIGS. 9a and 9 b are graphs showing Rockwell hardness of 3Cr-3WVTabase (VT alloys) with compositional variations.

[0029]FIG. 10 is a graph of yield stress of 3Cr-3WVTa base (VT alloys)with compositional variations.

[0030]FIG. 11 is a graph of yield stress of 20-lb AIM (V6) and VIM heatsof steel that do not contain tantalum (V steels).

[0031]FIG. 12 is a graph of Charpy curves for 20-lb VIM heats of the Vsteels.

[0032]FIG. 13 is a graph of yield stress of 20-lb AIM heats of steelthat contain tantalum (VT steels).

[0033]FIG. 14 is a graph of creep-rupture life of 20-lb AIM heats ofsteel that contain tantalum (VT steels).

[0034] For a better understanding of the present invention, togetherwith other and further objects, advantages and capabilities thereof,reference is made to the following disclosure and appended claims inconnection with the above-described drawings.

DETAILED DESCRIPTION OF THE INVENTION

[0035] The first series of studies on composition effects were conductedon small (500-g) experimental heats of steel. The steels were cast as≈1-in×0.5-in×5-in ingots that were subsequently rolled to 0.25-in. plateand 0.030-in. sheet, from which ⅓-size Charpy specimens and sheettensile specimens were machined, respectively. The steels were givendesignations that provide nominal composition for the major elements Cr,W, and Mo.

[0036] Unless otherwise stated, the other elements in the steels werefixed at the following nominal compositions: V at 0.25%, C at 0.1%, Taat 0.07-0.1%, Mn at 0.40-0.50%, Si at 0.1-0.2%, P at ≈0.015%, and S at0.008% (all compositions in wt. %). The designation of 3Cr-3WVTa thenspecifies as steel with nominal composition of Fe-3% Cr-3% W-0.25%V-0.1% Ta-0.45% Mn-0.15% Si-0.1% C with a small amount of impurities (P,S, etc.).

[0037]FIG. 3 shows a schematic representation of a continuous-coolingtransformation (CCT) diagram. If a steel is cooled at a rate that passesthrough Zone I, acicular bainite forms; if it passes through Zone II(and avoids the ferrite transformation regime), granular bainite forms;if it passes through Zone 3, soft ferrite forms.

[0038]FIG. 4 shows the microstructure of normalized (a) 3Cr-2WV and (b)3Cr-3WV steels with the desired acicular bainite achieved by increasinghardenability over that of the 2¼Cr-2WV. This microstructure wasobtained under the same conditions that produced granular bainite in2¼Cr-2WV.

[0039] The molybdenum and tungsten ranges were revised based partiallyon the tensile and Charpy data in Tables 1 and 2, respectively. Thetensile data shown in Table 1 indicate that increasing molybdenum in the3Cr-3WV steel from 0 to 0.25% and 0.5% in the presence of 3% and 2% W,respectively, causes an increase in the strength. A similar changeoccurs when 0.25% Mo is added to the 3Cr-3WVTa steel. The results forthe DBTT are shown in FIG. 5. TABLE 1 Yield Stress Data Showing theEffect of Molybdenum Yield Stress (Mpa) Tempered at 700° C. Tempered at750° C. Alloy Designation* RT 600° C. RT 600° C. 3Cr-3WV 797 614 577 4433Cr-3W-0.25MoV 821 567 595 474 3Cr-2W-0.5MoV 826 592 592 431 3Cr-3WVTa835 609 728 546 3Cr-3W-0.25MoVTa 935 641 675 403 3Cr-2W-0.75MoVTa 991ND** ND ND

[0040] TABLE 2 Charpy Impact Data Showing the Effect of MolybdenumTempered at 700° C. Tempered at 750° C. Untempered Alloy Designation*DBTT (° C.) USE (J) DBTT (° C.) USE (J) DBTT (° C.) USE (J) 3Cr-3WV −5910.0 −96 13.8 −28 8.1 3Cr-3W-0.25MoV −50 10.6 −113  11.8 −25 8.93Cr-2W-0.5MoV −80 11.0 −123  11.2 −63 8.0 3Cr-3WVTa −138  12.3 −98 12.4−64 11.0  3Cr-3W-0.25MoVTa −57  9.2 −84 10.2 −80 6.4

[0041]FIG. 5 shows the effect of varying the molybdenum composition onthe DBTT of 3Cr-3WV 15 and 3Cr-3WVTa steels.

[0042] These improvements in strength are accompanied by improvements inthe DBTT and USE in the Charpy tests shown in Table 2 for both the3Cr-3WV and 3Cr-3WVTa steels. (Note that all of the Charpy data in theseand many of the following tables are for miniature ⅓-size Charpyspecimens, and this is the reason for the small USE relative to that ofa standard Charpy specimen.) The improvement occurs in both thenormalized and the normalized-and-tempered conditions. The partialreplacement of tungsten by molybdenum appears to have more effect thanjust adding molybdenum to the 3% W steel.

[0043] What is especially important in the Charpy data is the decreasein the ductile-brittle transition temperature in the untemperedcondition, since it is the elimination of the time-consuming andexpensive tempering treatment that makes the new steels most attractiveto replace commercial steels in use presently. Tensile tests of a3Cr-2W-0.75MoVTa steel indicated a still higher room temperature yieldstress, although at 600° C., there was no improvement.

[0044] These results indicate that molybdenum in combination withtungsten can improve the properties of the 3Cr—WVTa steels over the useof tungsten by itself. However, it is necessary to limit the totalamount of the two elements, since these elements promote the formationof the undesirable Laves phase—Fe₂Mo, Fe₂W, or Fe₂(MoW). To minimizeLaves phase, the Mo and W will be limited as follows: 2[Mo]+[W]≦3.5,where [Mo] and [W] are compositional concentrations in wt. %.

[0045] Tables 3 and 4 compare the properties of a steel with 3% Cr, 3%W, and 0.4% V (a higher vanadium concentration than established in theoriginal patent) with the basic steel proposed in the previous patent,which contains 3% Cr, 3% W, and 0.25% V (3Cr-3WV). TABLE 3 Effect ofVanadium on Charpy Impact Properties Tempered at 700° C. Tempered at750° C. Untempered Alloy Designation* DBTT (° C.) USE (J) DBTT (° C.)USE (J) DBTT (° C.) USE (J) 3Cr-3W-0.25V  −59 10.0 −96 13.8 −28  8.13Cr-3W-0.4V −129 11.0 −96 11.1 −82 10.3

[0046] TABLE 4 Effect of Vanadium on Yield Stress Yield Stress (Mpa)Tempered at 700° C. Tempered at 750° C. Alloy Designation* RT 600° C. RT600° C. 3Cr-3W-0.25V 722 527 552 413 3Cr-3W-0.4V 781 540 565 403

[0047] Data in Table 3 show that increasing vanadium in the 3Cr-3WVsteel from 0.25 to 0.4 wt % decreases the DBTT in the untemperedcondition by the same amount that is produced by tempering the steel at750° C.—the highest tempering temperature used and the heat treatmentexpected to produced the best toughness. In addition to improving theDBTT, the increase in vanadium also improves the yield strength at bothroom temperature and 600° C., as shown in Table 4.

[0048] Comparison of data in Tables 2 and 3 indicates that improvementsin DBTT with an increase in vanadium from 0.25 to 0.4% are even greaterthan obtained with 2% W and 0.5% Mo. These results suggest that there ismore than one option to obtain a superior toughness/strength combinationin the Fe-3Cr-3W—V steels, especially for the steel to be used without atempering treatment.

[0049] One reason for widening the carbon concentration range is thatthe original work concentrated on the 0.1 wt % C steel (a typicalcomposition for these types of steel), and therefore, the range shouldhave been wider to allow a specification of a range of compositions forthe steel processors. Since then, more work on the steels producedanother reason for the range change as illustrated by the data in Table5. TABLE 5 Effect of tantalum on the Charpy Impact Properties Temperedat 700° C. Tempered at 750° C. Untempered Alloy Designation* DBTT (° C.)USE (J) DBTT (° C.) USE (J) DBTT (° C.) USE (J) 3Cr-3WV  −59 10.0 −9613.8 −28  8.2 3Cr-3WV-0.09Ta-0.08C −138 12.3 −98 12.4 −64 11.03Cr-3WV-0.05Ta-0.09C  −66  9.4 −103  11.8 ND 3Cr-3WV-0.17Ta-0.09C −11514.2 −91 13.2 −72 12.4

[0050] This table shows Charpy data for three steels with differenttantalum concentrations (0.05, 0.09 and 0.17 wt %) and the data for thebase steel. All of the tantalum-modified steels are improvements overthe base composition. Further, for the steels with 0.05 and 0.09% Ta,the properties of the steel with the lowest carbon concentration and thehighest tantalum had superior properties compared to that with lowertantalum and higher carbon. This implies that the tantalum and carboncompositions can be manipulated to optimize the properties. Thisoptimization could result in a steel with a carbon concentration lowerthan the 0.1 wt % level, a desirable result, because lower carbon meansimproved weldability. The yield stresses of the steels with 0.05 and0.09% Ta were comparable after the 700° C. temper, but the steel withthe 0.09% Ta had the best strength after the 750° C. anneal. Table 5also indicates that a higher Ta level leads to increased toughness.However, the steel with 0.17% Ta had lower strength than the other twosteels, implying that a balance needs to be achieved between the Ta andC, which will be discussed below.

[0051] Nickel is known to improve the toughness of ferritic steels, andthis was shown to be the case for the 3Cr-3WV steel, as shown in Table6. Therefore, nickel is being added to the composition specificationsfor this effect. Manganese has a similar effect. Since nickel is not tobe used for reduced-activation steels, for which the steels wereoriginally developed (see previous patent), the manganese range has beenexpanded for this purpose. TABLE 6 Effect of Nickel on the CharpyProperties Tempered at 700° C. Tempered at 750° C. Untempered AlloyDesignation* DBTT (° C.) USE (J) DBTT (° C.) USE (J) DBTT (° C.) USE (J)3Cr-3WV  −59 10.0  −96 13.8 −28 8.2 3Cr-3WV-2Ni −125 10.0 −148 11.2 ND

[0052] The new 3Cr steels are intended for elevated-temperatureapplications. Therefore, creep properties are important. Creep studieswere made on the base compositions discussed above, 3Cr-3WV and3Cr-3WVTa, on specimens taken from larger heats than those from whichthe above tests (1 lb) were taken. The heats were about 370 lb (168 kg)made by a vacuum-induction melting/vacuum-arc re-melt (VIM/VAR) process.Chemical compositions are given in Table 7. TABLE 7 Chemical Compositionof 370-lb VIM/VAR Heats of Steel (wt. %) Steel C Mn P S Si Cr V W N Ta3Cr-3WV 0.10 0.39 0.010 0.004 0.16 3.04 0.21 3.05 0.004 <0.01 3Cr-3WVTa0.10 0.41 0.011 0.005 0.16 3.02 0.21 3.07 0.003  0.09

[0053] The VIM/VAR heats were forged to bars 2×5×60 inches. To obtainthe test specimens, the steels were hot rolled to 0.625-in plate. Theplates were normalized by austenitizing 1 h at 1100 ° C., followed by anair cool. Some specimens were tested in the normalized condition, andother were in the normalized-and-tempered condition, where tempering ofthe plates was for 1 h at 700° C.

[0054] Creep-rupture studies of the 3Cr-3WV and 3Cr-3WVTa steels weremade at 600° C., as shown in FIG. 6 and 650° C., as shown in FIG. 7. Atboth temperatures, the results demonstrate the effect of tantalum onimproving the creep-rupture properties. The rupture lives of the3Cr-3WVTa were 2-3 times longer than those for the 3Cr-3WV steel at both600 and 650° C. For the 3Cr-3WV steel, there was a difference in theproperties of the steel in the normalized and thenormalized-and-tempered conditions. There was essentially no differencebetween the two different heat-treated conditions for the 3Cr-3WVTa.

[0055] The 3Cr-3WVTa steel had properties that were better than those ofsome of the commercial steels used for the applications for which thenew 3Cr steels are designed. These are T23, a nominalFe-2.25Cr-1.5W-0.2Mo-0.25V-0.005B-0.07C steel, T24, a nominalFe-2.4Cr-1Mo-0.25V-0.005B-0.07C steel, and T91, a nominalFe-9Cr-1Mo-0.2V-0.06Nb-0.06N-0.07C steel. For all three, the superiorityat 600° C. of the 3Cr-3WVTa is obvious. Referring to FIG. 7, at 650° C.,data for comparison were only available for the T91, and again the3Cr-3WVTa steel has better properties than those of the T91 at thistemperature.

[0056] The creep-rupture tests described hereinabove demonstrate thatthe base 3Cr-3WV and 3Cr-3WVTa steels have superior properties comparedto the commercial steels T23, T24, and T91. The 0.09% Ta addition to the3Cr-3WV composition has the effect of increasing the creep-rupturestrength by 2-3 times. Furthermore, the 3Cr-3WV and 3Cr-3WVTa can beused without tempering and still get improved creep strength over thecommercial steels, which are typically used in a tempered condition.

[0057]FIG. 6 shows creep-rupture properties of the 3Cr-3WV and 3Cr-3WVTasteels at 600° C. in the normalized and normalized-and-temperedconditions compared to three commercial steels. FIG. 7 showscreep-rupture properties of the 3Cr-3WV and 3Cr-3WVTa steels at 650° C.in the normalized and normalized-and-tempered conditions compared to acommercial steel.

[0058] The first tests on specimens from 1-lb (500-g) heats describedhereinabove indicated that steels with excellent tensile and impactproperties can be obtained if the steels have a base of3Cr-3W-0.25V-0.1C (3Cr-3WV) and 3Cr-3W-0.25V-0.10Ta-0.1C (3Cr-3WVTa) andcontain about 0.2Si and 0.5Mn. Creep-rupture studies on specimens from370-lb heats, described herein, were then made on the base compositions.To further delineate the optimum chemical composition of the steels,these base compositions were used as the starting point to examinevarying chemical compositions to determine the optimum composition rangefor the various elements to be included in the prospective steels.

[0059] The approximately 1-lb vacuum-arc heats and about 20-lb (9-kg)air-induction melted heats (AIM) and vacuum-induction melted (VIM) heatswere prepared. The small ingots (1 in×1 in×4 in) were hot rolled at1150° C. to 0.5-in thickness. The large heats (2.5 in×2.5 in×8 in) wereforged 25% at 1150° C. and then hot rolled at 1150° C. to 0.5-inthickness. The rolled plates were normalized (either 1100° C./1h/AC or1150° C./1h/AC) and tempered (700° C./1h/AC). For selected alloys,specimens were machined from the small heats for metallography, Rockwelland hot hardness (room temperature to 700° C.) tests, two tensile tests(one at room temperature and one at 650° C.), and room temperature and−40° C. Charpy tests (with a miniature specimen). Similar specimens wereobtained from the large heats (full-size Charpy specimens were obtained,in this case), and in addition, four creep specimens were obtained.

[0060] Compositions of the steels with the 3Cr-3WV (V alloys) as thebase composition are given in Table 8, and those with the 3Cr-3WVTa base(VT alloys) are given in Table 9. The V alloy, shown in Table 8, and theVT alloy, shown in Table 9 are the respective base compositions. TABLE 83Cr-3WV Steels With Varying Chemical Compositions (wt %)^(a) Steel C MnSi Cr V W Mo Ta Nb N B V^(b) 0.10 0.40 0.16 3.00 0.21 3.00 V1^(b) 0.101.00 1.00 3.00 0.21 3.00 0.05 V2^(b) 0.10 0.50 0.50 3.00 0.21 3.00 0.05V3^(b) 0.10 1.00 1.00 3.00 0.21 3.00 1.00 0.05 V4^(b) 0.10 0.50 0.503.00 0.21 3.00 1.00 0.05 V5^(b) 0.10 1.00 1.00 3.00 0.21 3.00 0.10 0.05V6^(c) 0.14 0.44 0.12 2.94 0.23 2.01 0.75  0.011 0.001 V6A^(d) 0.07 0.570.23 3.01 0.24 2.02 0.75 <0.001 0.001 V6B^(d) 0.07 0.46 0.22 3.01 0.242.03 0.75 <0.001 <0.001  V7^(d) 0.08 0.24 0.21 3.01 0.24 1.54 0.75<0.001 0.001 V7A^(d) 0.14 0.47 0.21 3.00 0.24 1.52 0.75 <0.001 V8^(d)0.13 0.27 0.21 3.04 0.24 1.55 0.76 <0.001 0.008 V8A^(d) 0.11 0.52 0.213.04 0.24 1.54 0.75 <0.001 0.007 V9^(d) 0.14 0.33 0.22 3.02 0.24 2.970.01 <0.001 0.001

[0061] TABLE 9 3Cr-3WVTa Steels With Varying Chemical Compositions (wt%)^(a) Steel C Mn Si Cr V W Mo Ta N B Hf Zr B VT^(b) 0.08 0.39 0.15 2.960.19 2.98 0.10 0.008 VT1^(b) 0.09 0.94 1.05 2.96 0.19 3.03 0.10 0.002VT2^(b) 0.09 0.39 0.16 2.97 0.20 3.04 0.24 0.001 VT3^(b) 0.10 0.40 0.163.00 0.21 3.00 0.50 VT5^(b) 0.10 0.40 0.16 3.00 0.21 3.00 2.00 VT6^(b)0.10 0.40 0.16 3.00 0.21 3.00 1.00 VT7^(b) 0.10 0.40 0.16 3.00 0.21 3.003.00 VT8^(b) 0.12 0.50 0.20 3.00 0.25 3.00 0.25 VT9^(b) 0.09 0.48 0.192.98 0.24 3.05 0.13 0.02  VT10^(b) 0.12 0.50 0.20 3.00 0.25 1.50 0.750.13 VT11^(b) 0.11 0.48 0.19 3.06 0.24 2.15 0.83 0.13 VT11A^(c) 0.120.39 0.15 2.99 0.23 2.06 0.75 0.036 0.01  VT11B^(c) 0.12 0.41 0.18 2.970.24 2.05 0.75 0.10 0.005 VT12^(b) 0.11 0.48 0.20 3.00 0.25 3.00 0.13VT12A^(c) 0.12 0.40 0.13 2.96 0.24 2.97 0.01 0.043 0.01  VT12B^(c) 0.120.56 0.19 2.96 0.24 2.98 0.01 0.13 0.005 VT13^(c) 0.11 0.43 0.13 2.950.23 2.01 0.74 0.04 0.013 0.001 VT14^(c) 0.12 0.44 0.13 2.95 0.23 2.000.75 0.05 0.01  0.005 VT14A^(d) 0.07 0.51 0.21 2.98 0.24 2.01 0.75 0.070.01  VT14B^(d) 0.07 0.51 0.21 2.98 0.24 2.01 0.75 0.07 0.008 VH^(b)0.12 0.50 0.20 3.00 0.25 2.99 0.13 VZ^(b) 0.12 0.50 0.20 3.00 0.25 2.990.07 VZA^(b) 0.12 0.50 0.20 3.00 0.25 3.00 0.13

[0062] Results for 1-lb Heats

[0063] For the small heats of V, as shown in FIG. 8 and VT, as shown inFIG. 9, the relative strength of the steels was first assessed byhardness. The V1-V4 steels with higher Si and Mn along with Nb, shown inTable 8 all had higher hardness than the base 3Cr-3WV (V) in thenormalized condition, and all but V4 were harder after tempering asshown in FIG. 8. Metallography indicated that V3 and V4 contained someferrite, probably because of the higher composition of ferriteformers—silicon and molybdenum. The niobium could also have an effect,if niobium carbides did not all dissolve during austenitization, thustying up the austenite former carbon and also reducing the hardenabilitywhen cooled, due to the reduced carbon in solution.

[0064]FIG. 8 shows Rockwell hardness of 3Cr-3WV base (V alloys) withvarious compositional variations, and FIG. 9 shows Rockwell hardness of3Cr-3WVTa base (VT alloys) with compositional variations.

[0065] Such an effect on hardenability was observed as shown in FIG. 9for tantalum for the 3Cr-3WVTa-base (VT) steels VT5 (2.0 Ta), VT6 (1.0Ta), and VT7 (3.0 Ta). In this case, the TaC did not dissolve duringaustenitization, and the hardenability was lower due to the lack ofcarbon in solution. This resulted in low hardness for these steels. Thesteel with 0.5% Ta (VT3) did not show a similar deterioration inhardness.

[0066] Both the V and the VT steels showed an effect of the combinationof 1% Mn and 1% Si.

[0067] The V1 (1% Mn, 1% Si) was harder than V and V2 (0.5% Mn, 0.5%Si), as shown in FIG. 8.

[0068] Likewise, the VT1 (1% Mn, 1% Si) was harder than the VT, as shownin FIG. 9. The hardness advantage was also observed for the tensileproperties, shown in Table 10. Despite the increase in strength for V1and VT1, there was also an increase in ductility for the stronger steelscontaining the larger amounts of Mn and Si. TABLE 10 Tensile Propertiesof the Experimental Steels Room Temperature Tests 650° C. Tests Steel YS(ksi) UTS (ksi) T. E. (%) ROA (%) YS (ksi) UTS (ksi) T. E. (%) OA (%)V^(a) 734  819 20.3 77.0 453 476 22.7 84.6 V1^(a) 880  965 17.4 70.9 502521 26.8 84.4 V6^(b) 979 1144 14.6 52.2 615 643 12.7 33.7 V6A^(c) 790 871 17.7 76.0 490 509 22.1 79.6 V6B^(c) 805  880 18.2 75.0 502 520 20.176.1 V7^(c) 764  834 17.9 78.2 468 485 19.9 82.2 V7A^(c) 833  938 18.769.0 504 527 20.9 80.3 V8^(c) 854  969 17.7 78.1 508 527 20.7 82.2V8A^(c) 846  987 15.8 65.3 553 583 21.0 76.6 V9^(c) 837  927 17.6 70.8494 512 25.8 80.6 VT^(a) 938 1064 17.8 60.8 540 553 13.7 72.2 VT1^(a)990 1114 17.5 62.4 564 603 22.7 74.2 VT2^(a) 937 1027 18.3 70.8 552 59120.5 77.0 VT8^(a) 953 1044 17.6 71.3 VT9^(a) 965 1078 14.6 58.9 587 62816.3 60.6 VT10^(a) 966 1077 17.2 68.1 586 620 18.4 78.0 VT11^(a) 9911110 16.4 65.7 602 640 17.6 76.9 VT11A^(b) 930 1017 17.8 63.4 573 60515.6 35.3 VT11B^(b) 1010  1122 15.1 64.4 614 632 13.7 50.6 VT12^(a) 9751073 17.6 67.3 570 606 19.8 78.5 VT12A^(b) 950 1046 15.5 57.2 563 58010.2 35.2 VT12B^(b) 975 1076 16.3 65.2 561 616 15.8 64.1 VT13^(b) 9181125 15.5 58.5 597 618 10.5 39.2 VT14^(b) 1011  1186 14.0 63.5 670 71413.3 47.0 VT14B^(c) 1024  1198 15.2 62.4 674 722 15.1 63.4 VH^(a) 9481056 17.6 68.7 565 601 16.1 68.6 VZ^(a) 902  992 17.6 72.2 509 531 17.576.6 VZA^(a) 725  804 15.9 66.4 425 440 21.5 78.6

[0069] A second series of small heats of the VT (VT8-VT12) steels wasprepared and tested as 10 shown in FIG. 9 to examine the effect of Ta(VT8 and VT12), Mo (VT10 and VT11), and N (VT9). There was relativelylittle difference between the hardnesses, especially in thenormalized-and-tempered condition, where the combination of 3.06% W and0.83% Mo (VT11) showed an advantage over the other steels. The tensiletests verified that there was not much difference between the steels, asshown in FIG. 10. The VT11 had the highest strength (just slightlyhigher than VT1) of these steels. Except for the steel with the 0.02% N,it also had the lowest ductility, as shown in Table 10. FIG. 10 showsyield stress of 3Cr-3WVTa base (VT alloys) with compositionalvariations.

[0070] Results for 20-lb Heats

[0071] The first 20-lb heats that were studied were prepared by AIM,after which the VIM process became available, as shown in Table 8. Forthe V steels (no tantalum), only one AIM heat was melted along withseveral VIM heats. The yield stress shown in Table 10 for the V6 (AIM),V6A, V6B, V7, V7A, V8, and V9 (VIM) heats indicate that the AIM heat(V6) is clearly stronger than the VIM heats, as shown FIG. 11. The V6steels contained 2.0% W, 0.75% Mo, the V7 and V8 steels contained 1.5%W, 0.75% Mo, and the V9 steel contained 3.0% W, 0% Mo. One possiblereason the V6 steel was stronger may be the nitrogen in this heat.However, the increase in strength comes at the expense of ductility, asshown in Table 10. For the VIM heats there is little difference. The V7Aand V8 appear somewhat stronger than the other VIM heats. These twosteels contain more carbon (0.13-0.14%) than that of the other threesteels (0.07-0.08%). The V8 also contains 0.008% B; this steel wasstronger than V7A at room temperature, but there was no difference at650° C. The relative change in the ultimate tensile strength was similarto that of the yield stress, as shown in Table 10. The ductilities ofthe VIM steels were also similar and considerably higher than that ofthe AIM heat (V6).

[0072]FIG. 12 shows the Charpy curves for the VIM V steels of FIG. 11.The V7, V7A, and V9 have similar curves, with the V7A having a slightadvantage, although this steel contains slightly less carbon than theother two steels. The V6A and V6B have similar properties at the highertemperatures, but they are quite different at the lowest temperatures.This despite the fact these steels contained carbon levels even lowerthan V7A. The V7 and V7A steels contained 1.5% W, 0.75% Mo, the V6A andV6B steels contained 2.0% W, 0.75% Mo, and the V9 contained 3.0% W andno molybdenum, thus indicating again there may be an advantage to thecombination of molybdenum and tungsten.

[0073] The first 20-lb heats produced for the VT steels were AIM heatsVT11A, VT11B, VT12A, VT12B VT13, and VT14, as shown in Table 9. Theyield stress of these steels showed only small variations, as shown inFIG. 13. At room temperature, VT11B was stronger than VT11A; thedifference is due to the tantalum content, with the VT11B containing0.10% Ta compared to 0.04% Ta for VT11A. A similar difference occurredfor the VT12A and VT12B, where the tantalum concentrations were 0.04 and0.13%, respectively. A comparison between VT11B and VT12B indicates thatthere is no benefit of the extra tantalum for the 0.13% Ta vs. 0.10% Ta.One other difference between the VT11A and B and the VT12A and B is thatthe former two contained 3% W and 0% Mo, whereas the latter twocontained 2% W and 0.75% Mo. The indication that VT11B is somewhatstronger than VT12B, even though the latter has more tantalum, arguesfor a strengthening effect for the combination of molybdenum andtungsten. The VT 13 and 14 also contain 2% W and 0.75% Mo, and they arealso stronger than the steels with just tungsten. The VT 14 alsocontained 0.01 B, and this steel was the strongest at both temperatures,even though it contained only 0.05 Ta. With the exception of the VT12B,however, the ductilities of these steels were quite low, especiallycompared to the 1-lb heats, as shown in Table 10. This is probably aneffect of the AIM vs. VIM techniques used for the 20-lb and 1 lb heats,respectively.

[0074]FIG. 11 shows yield stress of 20-lb AIM (V6) and VIM heats ofsteel that do not contain tantalum (V steels) and FIG. 12 shows Charpycurves for 20-lb VIM heats of the V steels.

[0075]FIG. 13 shows yield stress of 20-lb AIM heats of steel thatcontain tantalum (VT steels and FIG. 14 shows creep-rupture life of20-lb AIM heats of VT steels.

[0076] The creep-rupture behavior as shown in FIG. 14 of the VT steelsfor tests at 25 ksi at 650° C. and 55 ksi at 600° C. reflect thestrength behavior, as shown in FIG. 13. The steels with the lowesttantalum and no boron (VT11A, VT12A, and VT13) have the shortest rupturelives. The addition of boron to the steel with only 0.05Ta appears tocompensate for the lower tantalum. There again appears to be abeneficial effect of the combination of molybdenum and tungsten asopposed to tungsten alone (compare VT1 1A and VT13 with VT12A).

[0077] Although the preferred product in many cases is a carbide-freeacicular bainite, other useful austenite transformation products can bemade in accordance with the present invention. General examples ofaustenite transformation products are ferrite, bainite, and martensite.Formation thereof generally depends on the cooling rate employed afterthe austenitizing temperature is reached.

[0078] The new alloy compositions of the present invention are useful asstructural material for applications in the chemical, petrochemical,power generation, and steel industries. Advantages of using the alloysof the present invention include:

[0079] 1. reduced thicknesses of components by as much as 50%;

[0080] 2. potential for not requiring certain heat treatments such as,for example, tempering and/or post-weld heat treatment, which are highlyenergy intensive;

[0081] 3. reduced component fabrication and welding time;

[0082] 4. reduced use of welding consumables; and

[0083] 5. reduced cost of component with improved performance.

[0084] The alloys of the present invention can be used to fabricatesundry articles that can benefit from the superior properties of thesteel alloys described hereinabove. Articles can be formed by variousforming methods, including, but not limited to: casting, forging,rolling, welding, extruding, machining, and swaging. Examples ofarticles that can be fabricated from the alloys of the present inventioninclude, but are not limited to:

[0085] 1. Heat exchange equipment and the like, for example: heatexchangers; feed water heaters; condensers; evaporators; coolers;re-boilers; surface steam condensers; fired heaters; furnaces; crackers;and related piping, tubing, fittings, expansion joints; valves and otherpressure containment components used to connect heat exchange equipmentand the like to other process equipment.

[0086] 2. Columns, towers, and the like, for example: packed columns;tray columns; cracking towers; absorbing towers; drying towers; prilltowers; coke drums; and related piping, tubing, fittings, valves andother pressure containment components used to connect columns, towers,and the like to other process equipment.

[0087] 3. Pressure vessels, reactors, and the like, generally from{fraction (3/16)}to 20 in. thick, 18 in. to 40 ft. in diameter and up to300 ft long, including related piping, tubing, fittings, valves andother pressure containment components used to connect pressure vessels,reactors, and the like, to other process equipment.

[0088] 4. Tanks, storage vessels, and the like, for example: flat bottomtanks; elevated storage tanks; bins; silos; pool liners; spheres;cryogenic, single wall vessels; cryogenic, double wall vessels; andrelated piping, tubing, fittings, valves, and other pressure containmentcomponents used to connect tanks, storage vessels, and the like to otherprocess equipment.

[0089] 5. Equipment for power production, for example: power boilers;heating boilers; electric boilers; hot water heaters; heat recoverysteam generators; gas and steam turbines and associated components;generators and associated components; and related piping, tubingfittings, valves and other pressure containment components used toconnect various pressurized components.

[0090] 6. Equipment for metals production, for example: hoods; ladles;kettles; arc furnaces and continuous casting equipment components.

[0091] 7. Piping, conduit, tubing, and the like of sundry sizes andconfigurations, for example: piping from 1″ nominal pipe size to 50″outside diameter and ⅛″ to 4″ wall thickness; and tubing from ½″ outsidediameter to 16″ outside diameter and 0.049″ to 3″ wall thickness.

[0092] 8. Valves and valve components of sundry sizes andconfigurations, from very small to very large (50 to 150,000 lbs).

[0093] 9. Welding electrodes, for example, wire, strips, rods, and thelike of sundry sizes and configurations.

[0094] While there have been shown and described what is at presentconsidered the preferred embodiment of the invention, it will be obviousto those skilled in the art that various changes and modifications maybe made therein without departing from the scope of the invention asdefined by the appended claims.

What is claimed is:
 1. A high-strength, high-toughness steel alloycomprising about 2.5% to about 4% chromium, about 1.5% to less than 2%tungsten, about 0.1% to about 0.5% vanadium, and about 0.05% to 0.25%carbon with the balance iron, wherein the percentages are by totalweight of the composition, wherein said alloy is heated to anaustenitizing temperature and then cooled to produce an austenitetransformation product.
 2. A steel alloy in accordance with claim 1wherein said austenite transformation product comprises a carbide-freeacicular bainite.
 3. A steel alloy in accordance with claim 1 furthercomprising up to about 0.25% tantalum.
 4. A steel alloy in accordancewith claim 1 further comprising up to about 1.5% molybdenum, where 2[Mo]+[W]<3.5.
 5. A steel alloy in accordance with claim 1 furthercomprising up to about 2% nickel.
 6. A steel alloy in accordance withclaim 1 further comprising up to about 0.01% boron.
 7. A steel alloy inaccordance with claim 1 further comprising up to about 1.5% manganese.8. A steel alloy in accordance with claim 1 further comprising up toabout 1% silicon.
 10. A steel alloy in accordance with claim 1 furthercomprising up to about 0.2% hafnium.
 11. A steel alloy in accordancewith claim 1 further comprising up to about 0.2% zirconium.
 12. A steelalloy in accordance with claim 1 further comprising up to about 0.25%niobium.
 13. A steel alloy in accordance with claim 1 further comprisingup to about 0.25% copper.
 14. A steel alloy in accordance with claim 1further comprising up to about 0.2% titanium.
 15. A steel alloy inaccordance with claim 1 further comprising 3% Cr, 1.5 to 3% W, 0.0 to1.5% Mo, 0.0 to 0.25% V, 0.0% to 0.25% Ta, 0.0 to 0.01% B, and 0.1% C.16. A high-strength, high-toughness steel alloy comprising about 2.5% toabout 4% chromium, about 1.5% to about 3.5% tungsten, greater than 0.3%to about 0.5% vanadium, and about 0.05% to 0.25% carbon with the balanceiron, wherein the percentages are by total weight of the composition,wherein said alloy is heated to an austenitizing temperature and thencooled to produce an austenite transformation product.
 17. A steel alloyin accordance with claim 16 wherein said austenite transformationproduct comprises a carbide-free acicular bainite.
 18. A steel alloy inaccordance with claim 16 further comprising up to about 0.25% tantalum.19. A steel alloy in accordance with claim 16 further comprising up toabout 1.5% molybdenum, where 2 [Mo]+[W]<3.5.
 20. A steel alloy inaccordance with claim 16 further comprising up to about 2% nickel.
 21. Asteel alloy in accordance with claim 16 further comprising up to about0.01% boron.
 22. A steel alloy in accordance with claim 16 furthercomprising up to about 1.5% manganese.
 23. A steel alloy in accordancewith claim 16 further comprising up to about 1% silicon.
 24. A steelalloy in accordance with claim 16 further comprising up to about 0.08%nitrogen.
 25. A steel alloy in accordance with claim 16 furthercomprising up to about 0.2% hafnium.
 26. A steel alloy in accordancewith claim 16 further comprising up to about 0.2% zirconium.
 27. A steelalloy in accordance with claim 16 further comprising up to about 0.25%niobium.
 28. A steel alloy in accordance with claim 16 furthercomprising up to about 0.25% copper.
 29. A steel alloy in accordancewith claim 16 further comprising up to about 0.2% titanium.
 30. A steelalloy in accordance with claim 16 further comprising 3% Cr, 1.5 to 3% W,0.0 to 1.5% Mo, 0.0 to 0.25% V, 0.0% to 0.25% Ta, 0.0 to 0.01% B, and0.1% C.
 31. A steel alloy in accordance with any one of claims 1-30,inclusive, wherein said steel alloy is formed into an article.
 32. Asteel alloy in accordance with claim 31 wherein said article comprisesat least one of the group consisting of heat exchange equipment, column,tower, tank, storage vessel, pressure equipment, pressure vessel,reactor, equipment for metals production, piping, tubing, valve, valvecomponent, expansion joint, and welding material.
 33. A steel alloy inaccordance with any one of claims 1-30, inclusive, wherein said articlerequires no tempering treatment after being air cooled from theaustenitizing temperature.
 34. A steel alloy in accordance with any oneof claims 1-30, inclusive, wherein said article requires no temperingtreatment after being quenched in a liquid from the austenitizingtemperature.
 35. A steel alloy in accordance with any one of claims1-30, inclusive, wherein said article requires no heat treatment priorto being welded.
 36. A steel alloy in accordance with any one of claims1-30, inclusive, wherein said article requires no heat treatment afterbeing welded.
 37. A steel alloy in accordance with any one of claims1-30, inclusive, wherein said article requires no heat treatment afterfabrication thereof.
 38. A method of producing a high-strength,high-toughness steel composition comprising the steps of: a. forming abody of a ferritic steel composition comprising about 2.5% to about 4%chromium, about 1.5% to less than 2% tungsten, about 0.1% to about 0.5%vanadium, and about 0.05% to 0.25% carbon with the balance iron, whereinthe percentages are by total weight of the composition; b. heating saidcomposition to an austenitizing temperature for a predetermined lengthof time; and c. cooling said composition from the austenitizingtemperature at a rate to form an austenite transformationmicrostructure.
 39. A method in accordance with claim 38 wherein saidaustenite transformation microstructure comprises a carbide-freeacicular bainite microstructure.
 40. A method in accordance with claim38 wherein said austenitizing temperature is at least 1250° C. and saidaustenitizing time is at least 0.25 hour.
 41. A method in accordancewith claim 38 wherein said heating step further comprises heating thebody in a medium selected from the group consisting of air, vacuum, andan inert atmosphere such as argon or helium.
 42. A method in accordancewith claim 38 wherein said heating step further comprises air coolingsaid body after heating.
 43. A method in accordance with claim 38wherein said cooling step comprises quenching said body in a liquidafter heating.
 44. A method in accordance with claim 38 wherein saidcooling step further comprises cooling said composition from theaustenitization temperature.
 45. A method in accordance with claim 38further comprising the step of tempering said body after cooling.
 46. Amethod in accordance with claim 38 further comprising tempering saidbody after cooling at a temperature of less than or equal to about 780°C. for a time of up to 1 hour per inch of thickness of said body.
 47. Amethod in accordance with claim 38 wherein the composition includes 3%Cr, 3% W, 0.25% V, 0.0% to 0.25% Ta, 0.0 to 0.01% B, and 0.5-0.15% C.48. A method in accordance with claim 38 wherein the compositionincludes 3% Cr, 1.5 to 3% W, 0.0-0.75% Mo, 0.2 wt % to 1.0wt % Si, 0.2wt % to 1.5 wt % Mn, 0.25% V, 0.0% to 0.25% Ta, 0.0 to 0.01% B, and 0.1%C.
 49. A method in accordance with claim 38 further comprisingincreasing the hardenability of the composition by adding a minoralloying element selected from the group consisting of boron, titanium,tantalum, nickel, manganese, molybdenum, niobium, silicon, nitrogen, andcopper.
 50. A method of producing a high-strength high-toughness steelcomposition comprising the steps of: a. forming a body of a ferriticsteel composition comprising about 2.5% to about 4% chromium, about 1.5%to about 3.5% tungsten, greater than 0.3% to about 0.5% vanadium, andabout 0.05% to 0.25% carbon with the balance iron, wherein thepercentages are by total weight of the composition; b. heating saidcomposition to an austenitizing temperature for a predetermined lengthof time; and c. cooling said composition from the austenitizingtemperature at a rate to form an austenite transformationmicrostructure.
 51. A method in accordance with claim 50 wherein saidaustenite transformation microstructure comprises a carbide-freeacicular bainite microstructure.
 52. A method in accordance with claim50 wherein said austenitizing temperature is at least 1250° C. and saidaustenitizing time is at least 0.25 hour.
 53. A method in accordancewith claim 50 wherein said heating step further comprises heating thebody in a medium selected from the group consisting of air, vacuum, andan inert atmosphere such as argon or helium.
 54. A method in accordancewith claim 50 wherein said cooling step further comprises air coolingsaid body after heating.
 55. A method in accordance with claim 50wherein said cooling step comprises quenching said body in a liquidafter heating.
 56. A method in accordance with claim 50 wherein saidaustenitizing step further comprises cooling said composition from theaustenitization temperature.
 57. A method in accordance with claim 50further comprising the step of tempering said body after cooling.
 58. Amethod in accordance with claim 50 further comprising tempering saidbody after cooling at a temperature of less than or equal to about 780°C. for a time of up to 1 hour per inch of thickness of said body.
 59. Amethod in accordance with claim 50 wherein the composition includes 3%Cr, 3% W, 0.25% V, 0.0% to 0.25% Ta, 0.0 to 0.01% B, and 0.5-0.15% C.60. A method in accordance with claim 50 wherein the compositionincludes 3% Cr, 1.5 to 3% W, 0.0-0.75% Mo, 0.2 wt % to 1.0 wt % Si, 0.2wt % to 1.5 wt % Mn, 0.25% V, 0.0% to 0.25% Ta, 0.0 to 0.01% B, and 0.1%C.
 61. A method in accordance with claim 50 further comprisingincreasing the hardenability of the composition by adding a minoralloying element selected from the group consisting of boron, titanium,tantalum, nickel, manganese, molybdenum, niobium, silicon, nitrogen, andcopper.
 62. A method of producing a high-strength, high-toughness steelalloy comprising the steps of: a. forming a body of a ferritic steelcomposition comprising 2.5% to 4.0% chromium, 1.5% to less than 2%tungsten, 0.0% to 1.5% molybdenum, 0.10% to 0.5% vanadium, 0.2% to 1.0%silicon, 0.2% to 1.5% manganese, 0.0% to 2.0% nickel, 0.0% to 0.25%tantalum, 0.05% to 0.25% carbon, 0.0% to 0.01% boron, 0.0% to 0.2%titanium, 0.05% to 0.25% Nb, 0.0% to 0.08 % nitrogen, 0.0% to 0.2% Hf,0.0% to 0.2% Zr, and 0.0% to 0.25% Cu, with the balance iron, whereinthe percentages are by total weight of the composition; b. heating saidcomposition to an austenitizing temperature for a predetermined lengthof time; and c. cooling said composition at a rate to form acarbide-free acicular bainite microstructure.
 63. The method of claim 62further comprising the additional step of: d. tempering said compositionat a temperature of not more than about 780° C. for a time of up to 1hour per inch of thickness of said composition.
 64. The method of claim62, wherein said cooling step comprises air cooling said composition.65. The method of claim 62, wherein said cooling step comprisesquenching said composition.
 66. A high-strength, high-toughness ferriticsteel article made according to the method of claim
 62. 67. A method ofproducing a high-strength, high-toughness ferritic steel alloycomprising the steps of: a. forming a body of a ferritic steelcomposition comprising 2.5% to 4.0% chromium, 1.5% to 3.5% tungsten,0.0% to 1.5% molybdenum, greater than 0.3% to 0.5% vanadium, 0.2% to1.0% silicon, 0.2% to 1.5% manganese, 0.0% to 2.0% nickel, 0.0% to 0.25%tantalum, 0.05% to 0.25% carbon, 0.0% to 0.01% boron, 0.0% to 0.2%titanium, 0.05% to 0.25% Nb, 0.0% to 0.08% nitrogen, 0.0% to 0.2% Hf,0.0% to 0.2% Zr, and 0.0% to 0.25% Cu, with the balance iron, whereinthe percentages are by total weight of the composition; b. heating saidcomposition to an austenitizing temperature for a predetermined lengthof time; c. cooling said composition at a rate to form a carbide-freeacicular bainite microstructure; and
 68. The method of claim 67 furthercomprising the additional step of: d. tempering said composition at atemperature of not more than about 780° C. for a time of up to 1 hourper inch of thickness of said composition.
 69. The method of claim 67,wherein said cooling step comprises air cooling said composition. 70.The method of claim 67, wherein said cooling step comprises quenchingsaid composition.
 71. A high-strength, high-toughness ferritic steelarticle made according to the method of claim 67.